Focusing servo controlling apparatus

ABSTRACT

A magneto-optical disc recording or reproducing device focusing control servo system which uses a digital signal processor (DSP) to control a pulse width modulation (PWM) circuit so that if the focusing servo is to be in operation alone, such as during a focusing search or seek, fixed clocks of a fixed frequency are used for processing focusing servo signals and after engagement of the focusing servo, focusing servo is effected responsive to focusing servo signals which are formed based upon a reproduced servo pattern of the magneto-optical disc.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a servo signal processing device provided in arecording or reproducing apparatus for an optical disc of a sample servosystem, such as a magneto-optical disc, a compact disc or a write-oncedisc. More particularly, it relates to a focusing servo controllingdevice in which, when effecting sample servo by itself, such as during afocusing search, the output timing of the focusing servo signals isprompted for improving the phase allowance of the focusing servo loopfor stabilizing focusing servo.

2. Description of the Related Art

The optical disc of the type having a sample servo system is well known.With the optical disc of this sample servo type, pre-set servo patternsare provided at a pre-set interval on the recording track. Recordingdata is recorded or reproduced between servo patterns as servo controlis performed based upon playback signals of the servo patterns. Theoptical disc reproducing apparatus reproducing recorded data from anoptical disc of the sample servo system is provided with a digitalsignal processor (DSP) for digitally processing servo error signals foreffecting the servo control.

If reproduction of the recorded data is specified, the optical discreproducing apparatus causes a laser beam to be illuminated on theoptical disc via an optical system. Since the servo patterns are formedat a pre-set interval on the optical disc, the laser beam is irradiatedon a servo area having the servo patterns recorded thereon and on a dataarea having recording data recorded thereon, in this order, as shown inFIG. 21a. One segment is made up of one servo area and one data area,such that the optical disc reproducing apparatus reproduces the recordeddata with plural segments as one reproducing unit.

The reflected light is produced on irradiating the optical disc with thelaser beam. The optical system receives this reflected light andaccordingly forms RF signals while also forming tracking error signalsand focusing error signals. These signals are sent to an A/D converterwhich is also fed with channel clocks generated by a phase locked loop(PLL) circuit based upon the repetition frequency of the servo patterns.Specifically, 216 channel clocks are formed for each segment. The A/Dconverter digitizes the RF signals, tracking signals and focusingsignals based upon these channel clocks for forming and outputting RFdata, tracking error data and focusing error data.

The RF data are supplied to a recording data detection system which thensamples the RF data with the channel clocks of a pre-set frequency toreproduce recorded data and sends the reproduced data to e.g., anexternal equipment, such as a computer or a speaker. On the other hand,the tracking error data and the focusing error data are latched andoutputted at the timing of reproduction of the servo area shown in FIG.21a by a tracking error latch pulse and a focusing error latch pulseshown in FIGS. 21b and 21c, respectively.

The DSP is configured for operating based upon a software programroutine, and captures latched tracking error data and focusing errordata based upon sampling interrupt pulses supplied at the output timingof the latched error data, as shown in FIG. 21d. For correctlyreproducing the recorded data, the tracking error data needs to becorrected more speedily than the focusing error signals. Consequently,the DSP processes the tracking error data, captured based upon thesampling interrupt pulses, before processing the focusing error data,similarly captured based upon the sampling interrupt pulses, as shown inFIG. 21e.

The DSP is fed with the channel clocks and counts the number of thechannel clocks for each segment. The DSP outputs the tracking errordata, processed as described above, at a timing when the count valuereaches 128, or "80h" in hexadecimal notation, as shown in FIG. 21f. Onthe other hand, the DSP outputs the focusing error data, processed asdescribed above, at a timing when the count value reaches 160, or "A0h"in hexadecimal notation, as shown in FIG. 21g.

The tracking error data and the focusing error data are supplied torespective servo systems. The servo systems effect phase compensation onthe tracking error data and the focusing error data and accordinglyservo-control the optical system in a direction of correcting thetracking errors and the focusing errors. This assures correctreproduction of recorded data in the "just-tracking" and in the"just-focusing" states at all times.

With the optical disc reproducing apparatus for reproducing the opticaldisc of the sample servo system, the tracking servo and the focusingservo are collectively controlled based upon the playback signals of theintermittently reproduced servo patterns. However, since it is necessaryto improve response to tracking errors, tracking error data is processedand outputted in advance of the focusing error signals. If only thefocusing servo is to be in operation, such as during focusing search orseek for effecting tentative focusing on the disc surface in advance ofreproduction, the tracking servo is not required.

However, if the focusing error data is outputted in such a case at thesame timing, it takes a lot of time from the time of sampling by the A/Dconverter until outputting of focusing error data from the DSP. Theresult is limitations imposed on the phase allowance of the servo loopand unstable servo states immediately after focusing servo engagement.Thus it may occur that a prolonged time elapses until the servo statebecomes stabilized and, in the worst case, the focusing servo, oncecaptured, is disengaged.

In view of this state of the prior art, it is an object of the presentinvention to provide a servo signal processing device in which, if thefocusing servo is to be in operation by itself with the use of theoptical disc of the sample servo system, the focusing error data isoutputted more promptly for improving the phase allowance of thefocusing servo loop for stabilizing the focusing servo.

Moreover, if the focusing servo is to be in operation by itself, e.g.during focus search or seek when tentative focusing is done on theoptical disc surface prior to reproduction, the phase locked loop (PLL)circuit for the focusing servo becomes an open loop and is brought tothe free running state. Thus, the frequency of the clocks for thefocusing servo, outputted from the voltage controlled oscillator (VCO),is varied by about 20%. On the other hand, it takes from several msec totens of msec from the time the focusing servo is engaged until the PLLcircuit is locked. Consequently, if the focusing servo is captured undersuch conditions to close the PLL circuit to effect phase compensation ofthe focusing servo, arithmetic operations for the phase compensationfilter become inaccurate such that accurate phase compensation becomesimpossible. This problem is most acute directly after focusing servocapture when the focusing actuator amplitude is larger and hence theservo state becomes unstable and the focusing servo, once engaged, tendsto be disengaged again even after phase compensation.

In view of the state of the prior art, it is another object of thepresent invention to provide a focusing servo controlling device inwhich, if the focusing servo is to be in operation by itself with theuse of the optical disc of the sample servo system, phase compensationdirectly after focusing servo capture is accurate to enable stablefocusing servo to be achieved.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provideda servo signal processing device in which at least tracking servosignals and focusing servo signals formed based upon playback output ofa pre-set servo pattern intermittently reproduced from an optical discon which the servo patterns are arrayed at a pre-set interval areengaged and outputted at a preset timing in the order of the trackingservo signal and the focusing servo signal, wherein the improvementcomprises output timing control means for controlling the output timingof the focusing servo signals so that, when the focusing servo iseffected by itself, the output timing is prompted at a pre-set time foroutputting only the focusing servo signals.

According to a second aspect of the present invention, there is provideda focusing servo controlling device of a phase-locked loop configurationin which the focusing servo is effected based upon focusing servo clocksformed by a playback output of pre-set servo patterns from an opticaldisc on which the servo patterns are arrayed at pre-set intervals.

The focusing servo controlling device includes fixed clock outputtingmeans for outputting fixed clocks for a focusing servo of a fixedfrequency, switching means for switching between outputting the focusingservo clocks or the fixed clocks, and control means for effectingswitching control of the switching means for selecting and outputtingthe fixed clocks when effecting the focusing servo alone and foreffecting switching control of the switching means for selecting andoutputting the focusing servo clocks directly after engagement of thefocusing servo.

With the focusing servo controlling device of the present invention, thecontrol means effect switching control of the switching means forselecting and outputting the fixed clocks, when effecting the focusingservo alone, while effecting switching control of the switching meansfor selecting and outputting the focusing servo clocks directly afterengagement of the focusing servo, thereby stabilizing the servo stateduring the focusing servo capture.

With the servo signal controlling device according to the presentinvention, the output timing of the focusing servo signal is controlledso that, when the output timing control means effects only the focusingservo, the output timing is prompted at a pre-set time so that only thefocusing servo signals will be outputted.

The foregoing and other objectives, features and advantages of theinvention will be more readily understood upon consideration of thefollowing detailed description of certain preferred embodiments of theinvention, taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a recording/reproducing apparatus for amagneto-optical disc employing a servo signal processing device of thepresent invention.

FIG. 2 is a diagram of a segment construction of a magneto-optical discemployed for recording/reproduction in the recording/reproducingapparatus of FIG. 1.

FIGS. 3A-3D are diagrams for explaining a format of a servo area of themagneto-optical disc.

FIG. 4 illustrates how the first pit in the servo area of themagneto-optical disc is detected.

FIGS. 5A-5C are diagrams for use in explaining a format of a servo areaof an access code recorded in an address segment of the magneto-opticaldisc.

FIG. 6 illustrates an example of the access code.

FIG. 7 is a diagram of the format of a data segment of themagneto-optical disc.

FIG. 8 is a diagram of a format of a servo area of a ROM disc.

FIG. 9 is a diagram of a reference pattern of a data sector of themagneto-optical disc.

FIG. 10 is a diagram of a setting parameter for zone division in themagneto-optical disc.

FIG. 11 is a diagram of the state of zone division in themagneto-optical disc.

FIG. 12 is a diagram of a sector format in the magneto-optical disc.

FIG. 13 is a diagram of the arraying state of GCP segments in themagneto-optical disc.

FIG. 14 is a diagram of the construction of the GCP segment.

FIG. 15 is a flow chart for illustrating the operation during a focusingsearch of the recording/reproducing apparatus for the magneto-opticaldisc.

FIG. 16 is a block diagram of a quartz oscillator and a 1/2 frequencydivider provided in a controller of the recording/reproducing apparatusfor the magneto-optical disc.

FIG. 17 is a block diagram of a 5-tuple circuit and a 1/4 frequencydivider provided in a DSP of the recording/reproducing apparatus for themagneto-optical disc.

FIG. 18 is a block diagram of a changeover switch of fixed clocks fromthe DSP and servo clocks from an SPLL and which is provided in the STGof the recording/reproducing apparatus for the magneto-optical disc.

FIGS. 19(a) to 19(e) are timing charts for illustrating a focusing servoduring focusing search and servo control of the recording/reproducingapparatus for the magneto-optical disc.

FIG. 20 is a flow chart for illustrating the operation of therecording/reproducing apparatus for the magneto-optical disc duringseek.

FIGS. 21(a) to 21(g) are timing charts for illustrating the operation ofperforming both tracking servo and focusing servo in therecording/reproducing apparatus for the magneto-optical disc.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings, a preferred embodiment of a focusing servocontrolling device according to the present invention will be describedin detail.

The focusing servo signal controlling device according to the presentinvention may be applied to a recording/reproducing apparatus for amagneto-optical disc shown in FIG. 1. The recording/reproducingapparatus for a magneto-optical disc is of the sample servo system andis made up of a disc drive 1 and a control block 2. Commands and dataare exchanged with a host computer 3 over a SCSI bus 4 connectedthereto.

The disc drive 1 includes a spindle motor 6 for rotating themagneto-optical disc 5 at a constant angular velocity and a spindledriver 7 rotating the spindle motor 6. The disc drive 1 also includes aloading mechanism 8 for loading the magneto-optical disc 5 within thedisc drive 1, and a pickup 9 for radiating a laser beam to themagneto-optical disc 5. The disc drive 1 also includes a laser diode 10for radiating a laser beam on the magneto-optical disc 5 and a laserdriver 11 for exciting the laser diode 10 for radiating the laser beamof a constant recording level during data recording and for radiatingthe laser beam of a constant reproducing level during data reproduction.

The disc drive 1 includes a photodetector 12 for receiving reflectedlight of the laser beam radiated on the magneto-optical disc 5 forgenerating RF signals, tracking error signals and focusing errorsignals, and a current-to-voltage converting block (IV converting block)13 for converting the RF signals supplied from the photodetector 12 ascurrent into voltage, amplifying the resulting voltage at a pre-set gainand outputting the resulting amplified voltage. The disc drive 1 alsoincludes a magnetic head 14 for applying a magnetic field modulated inaccordance with data during data recording, and a magnetic head driver15 for driving the magnetic head 14 in accordance with recording data.

The control block 2 is constructed by connecting a data processing block25 and a servo control block 26 via a digital signal processor (DSP) bus22 and a central processing unit (CPU) bus 34 to a controller 21. Thehost computer 3 is connected via the small computer system interface(SCSI) bus 4 to the controller 21.

The data processing block 25 has an input/output (I/O) block 23 which isfed with front auto-power control (FAPC) data for controlling the laserlevel of the laser beam radiated on the magneto-optical disc 5,depending upon the laser level detected in the forward path of the laserbeam radiated on the magneto-optical disc 5, and a D/A converter 24 forconverting the FAPC data into corresponding analog signals (LDbias)which are supplied to the laser driver 11.

The data processing block 25 also includes a selector and clampingcircuit 27 for selecting and clamping the magneto-optical (MO) signaland the RF signal from the IV converting block 13 of the disc drive 1based upon timing clocks for a servo control system from a servo systemtiming generator (STG) 32 as later explained, a servo clock generatingcircuit (SPLL) 29 of a phase locked configuration for generating servoclocks (sclk) and a data clock circuit (DPLL) 30 of a phase lockedconfiguration for generating data clocks (dclk).

The data processing block 25 further includes a clock selector 31 forselecting the servo clocks (sclk) or the data clocks (dclk) based upontiming clocks from the STG 32 and an A/D converter 28 for digitizing theRF signal or the MO signal from the selector and clamping circuit 27with clocks selected by the clock selector 31. The data processing block25 also includes a data system timing generator (DTG) 35 for formingdata-processing timing clocks based upon data clocks (dclk) from theDPLL 30, and a recording/reproducing (R/R) circuit 36 for supplyingrecording data (wdat) to the magnetic head driver 15 based upon dataclocks (dclk) from the DPLL 30 and data-processing timing clocks fromthe DTG 35 and for supplying reproduced data via the controller 21 tothe host computer 3.

The servo control block 26 includes a multiplexor (MUX) 41 for selectingand outputting the focusing error signals, FAPC signals and trackingerror signals from the IV converting block 13 of the disc drive 1, anA/D converter 42 for digitizing the signal from the multiplexor 41 basedupon servo clocks from the SPLL 29 (via the STG 32), an I/O block 43 fedwith A/D converted data and a digital signal processor (DSP) 47 forexecuting digital signal processing of the recording/reproducingapparatus for the magneto-optical disc.

The servo control block 26 also includes an I/O block 46 to and fromwhich control data for controlling the rotation of the spindle motor 6of the disc drive 1 are entered or outputted, a pickup driver 44 forcontrolling the focusing and tracking of the pickup 9, and a pulse widthmodulation (PWM) circuit 45 for pulse width modulation driving of thepickup driver 44. The servo control block 26 further includes a focusingtiming control register 48 for setting the output timing control data ofthe focusing error data by the DSP 47, a counter 49 for counting thenumber of servo clocks from the SPLL 29, and a comparator 50 forcontrolling the PWM circuit 45 so that focusing error data will beoutputted by the PWM circuit 45 at a timing of coincidence between thecount value specified by data set in the focusing timing controlregister 48 and the count value from the counter 49.

The magneto-optical disc 5 employed in the recording/reproducingapparatus has a complete turn of a track divided into 1400 segments(0-1399), as shown in FIG. 2. These segments are classified into twokinds of segments: an address segment ASEG and a data segment DSEG.

In the address segment ASEG, the position information in the radialdirection of the disc and that in the tangential direction of the discare recorded as pre-pits. The address segment is provided every 14segments, so that 100 address segments are provided in one turn of thetrack. Each frame is formed from one address segment, e.g. ASEG 0, tothe next address segment, e.g. ASEG 14. There are address segments ASEGfor 100 frames in one complete turn of the track, and 13 data segmentsDSEG are formed between any two address segments ASEG. Thus, there are1300 data segments DSEG in one complete turn. Each segment, whether anaddress segment ASEG or a data segment DSEG, is made up of a servo areaARs having a duration of 24 clock cycles and a data area ARd having aduration of 192 clock cycles (for a total of 216 clock cycles).

The data portion of the address segment ASEG includes an address sectionARda which contains address information and a laser control section Ardbwhich is used by the disk drive components shown in FIG. 1 to controlvarious operation parameters of the laser beam used to scan themagneto-optical disk. The format of the servo area ARs of the addresssegment and of the different types of data segments is shown in FIGS.3A-3D. Each servo area contains three pre-recorded pits, identified as asegment mark pit Pa and wobble pits Pb and Pc. The positions of thewobble pits are fixed and the position of the segment mark pit Pa isdependent upon the particular type of segment in which that segment markpit Pa is recorded. Referring first to the wobble pits Pb and Pc, thesepits are located at clock cycles 11 and 16 and are offset from thecenter line of the track by, for example, 1/4 of the track pitch, asshown in FIGS. 3A-3D. These wobble pits are used for tracking control.It is appreciated that if the scanning laser beam is offset, ordisplaced from the center line of the track, the signal produced fromone of the wobble pits Pb or Pc will be greater than the other. Thus, anindication of the direction and intensity of a tracking error isproduced and may be readily compensated. In addition, since thepositions of the wobble pits Pb and Pc are fixed, pulses produced byscanning these pits will exhibit a frequency determined by the speed ofthe disk, thereby permitting a servo clock (from which system clockpulses are generated) to be synchronized therewith.

As shown in FIGS. 3A-3D, each servo area in each segment includes asection ARfs in which no pits are recorded. The focus sample area ARfsof the servo area ARs is a mirror portion and is used for the focusingservo, read power APC (automatic power control) or RF signal clamping.The positions of various sample pulses for theses operations aredifficult to identify correctly, such that variations not larger than ±5servo clocks may be estimated. The format of the pits shown in FIGS.3A-3D is preferred because it minimizes that portion of the disk inwhich no pits are formed, known as the mirror portion, thereby reducingthe possibility that "ghost pits" will be produced during the diskmolding process. It is preferred, therefore, to record each pit with aduration of two clock cycles and to separate the pits in the servoportion by at least 5 clock cycles. Consequently, inter-pit interferencemay be kept desirably small.

The position at which the segment mark pit Pa is recorded in the leadingsection of each servo portion identifies the segment as either anaddress segment ASEG or a data segment DSEG. If it is a data segment,the position of the segment mark pit Pa also identifies the data segmentas the first data segment in a sector, the last data segment in thesector, or any other intermediate data segment in the sector. The numberof segments in a sector is dependent upon the radial position of thatsector. Referring to FIGS. 3B-3D and 4, if the segment mark pit Pa isrecorded in clock cycles 3-4 of a segment, that pit (ADDRESS MARK)identifies the segment as an address segment ASEG. If the segment markpit Pa is located at clock cycles 4-5, the pit (SECTOR MARK 1)identifies the segment as a data segment DSEG II, and moreover, as theleading, or first, data segment in the sector. If the segment mark pitPa is located at clock cycles 5-6, the pit (SECTOR MARK 2) identifiesthe segment as the last data segment DSEG III of a sector. Finally, ifthe segment mark pit Pa is located at clock cycles 6-7, the pit (SEGMENTMARK) identifies the segment as any data segment DSEG I, other than thefirst or last, in the sector. The presence and position of a segmentmark pit Pa are determined by generating sampling pulses at those clockcycles identified as A, B, C and D, shown in FIGS. 3A-3D, and 4. Thus,if a segment mark pit Pa is sensed by sampling pulse A, the segment inwhich that segment mark pit is located is identified as an addresssegment. If the segment mark pit is sensed by sampling pulse B, thesegment is detected as the first segment of a sector. If the segmentmark pit Pa is sensed by sampling pulse C, the segment is detected asthe last segment of a sector. Further, if the segment mark pit Pa issensed by sampling pulse D, the segment is detected as any intermediatesegment in the sector.

It is appreciated that since the identification of a segment is readilydetermined simply by sensing the position of the segment mark pit Pa, itis not necessary to provide substantial segment identifying data whichmight detract from the amount of data that can be recorded in the dataportion of each segment. As a result, the servo portion can be madesmaller and information is recorded with higher efficiency.

FIGS. 5A-5C represent the information that is recorded in an addresssegment ASEG. The construction of the address information included inthe data portion of the address segment is shown in FIG. 5B and includesan access code and a frame code. The access code includes a 16-bit trackaddress formed of 4-bit groups AM, A2, A3, and AL together with a paritycode P. This track address is recorded in Gray code and identifies theradial position of the address segment. The frame code identifies theframe address FM and FL, thereby identifying the frame in which theaddress segment is recorded, and this identifies the circular positionof that frame in a turn. The track addresses and the frame addresses arerecorded in Gray code, as illustrated in FIG. 6 which shows the trackaddresses in Gray code.

The access code is formed of 4-bit groups in which the 1's complement ofeach group is converted. The value of the 4-bit group AM is consideredthe most significant value or number (MSN) and the value of the 4-bitgroup AL is considered the least significant number (LSN). The 1'scomplement conversion is carried out in this order so that the accesscodes are changed by only one pattern across neighboring turns ortracks. Similarly, the frame code is an 8-bit frame address formed oftwo 4-bit groups FM and FL. Here too, the frame code is recorded as Graycode information.

Referring now to FIG. 7, the data area ARd of the data segment DSEG ismade up of the data area ARda of 176 to 376 data clocks for recordinguser data, a pre-write area ARpr of 12 data clocks, and a post-writearea ARps of four data clocks. The pre-write area ARpr is provided forsecuring a distance required for pre-heating after laser irradiationuntil stabilization of the disc temperature and for use as a clamp areafor suppressing DC fluctuations caused by double refraction of MOsignals. The post-write area ARps is provided for assuring a distancefor eliminating insufficient erasure of recorded data and for avoidinginterference otherwise caused by the edge of a groove Gr. The groove Gris formed in the data area ARd and is not employed for tracking andhence it is not required to be precise in depth. By providing the grooveGr, the mirror portion as required may be decreased to alleviate illeffects on servo pits caused due to disc molding.

The MO disc is unidirectionally bulk-erased at the time of shipment, sothat the disc may be used without requiring a formatting operation. Byrecording data of the same magnetic properties as those in the bulkerasure direction on the pre-write area ARpr, recorded data remainsunchanged even if data cannot be correctly recorded on the pre-writearea ARpr due to insufficient residual heat of the recording medium, sothat a stable signal may be reproduced.

The foregoing description refers to a magneto-optical disc 5 forrecording/reproduction. If the disc is a replay-only ROM disc, 3-clockduration anchor pits Pan are provided at the leading end of thepre-write area ARpr as shown in the upper right corner of FIG. 8, sothat the mirror portion as required may be decreased to alleviate anyill effects on servo pits due to the disc molding process.

Each data sector is made up of 66 bytes of reference data, 2048 bytes ofuser data (D0 to D2047), 256 bytes of ECC (E1,1 to E16, 16), 8 bytes ofCRC (CRC 1 to CRC 8), 8 vender unique bytes and 32 bytes of user defineddata (UD), totaling at 2418 bytes. Referring to FIG. 9, 66 bytes ofspecified patterns, made up of four blocks, each consisting of 4 bytesof 8T patterns and 12 bytes of 2T patterns, and 2 bytes of all 0s asallowance for setting the detected information, are recorded as thereference data. The 8T pattern is employed for setting the three-valuelevel (high H, mid M and low L) for data detection, while the 2T patternis used for correcting the DC-derived pit position shift, caused bye.g., recording power variation, during reproduction. In the data areaARd of the data segment DSEG, data other than the 66 bytes of referencedata are scrambled, and the scrambled data are NRZI converted andrecorded on the segment basis.

As shown in FIGS. 10 and 11, this optical disc is a zone CAV disc, andis made up from the outer rim part of a GCP (Gray Code encoded Part ofthe control track) band of 736 tracks, a 2-track buffer track, a 5-trackcontrol track, a 2-track buffer track, a 5-track test track, an 848track user zone 0, an 864-track user zone 1, an 880-track user zone 2, a912-track user zone 3, a 944-track user zone 4, a 976-track user zone 5,a 1024-track user zone 6, a 1056 track user zone 7, an 1120 track userzone 8, an 1184 track user zone 9, a 1216 track user zone 10, a 1296track user zone 11, a 1392 track user zone 12, a 1488 track user zone13, a 1696 track user zone 14, a 770 track user zone 15, a 5-track testtrack, a 2-track buffer track, a 5-track control track, a 2-track buffertrack and an 820-track GCP band.

If, with the number of tracks (TRACzone) in a zone, the number of datasegments (DSEGsect-zone) required for a sector in a zone, and the numberof data segments per track (DSEGtrack), the sector is to be completedfrom zone to zone and the number of sectors is to be constant, itsuffices to determine the number of tracks so that the number of sectors(SCTzone) in a zone is:

    SCTzone=TRACzone*DSEGtrack/DSEGsect-zone, and

    TRACzone=K*DSEGsect-zone,

where K is the total number of zones.

All parameters may be obtained by allocating the approximate datacapacity per zone, obtained on dividing the data capacity of the entiredisc by K, from the outer rim side zone, and by determining the dataclock frequency so that the recording density of the innermost track ofthe zone will not fall below a pre-set density.

If, in this case, a sector is started from a segment, the number ofsegments making up the sector and the sector are terminated and, even ifthere exist redundant bytes in the last segment, the next sector isstarted with the next segment. Thus, at the leading end of a zone, asector necessarily starts with a 0 frame code sector. In the innermostzone, the number of sectors is not necessarily the same as the number ofsectors in the remaining zones, but a fractional sector is likely to beproduced. However, the innermost zone may be terminated at a track whosesector ends at the segment 0 for facilitating calculation of the paritysector capacity.

With the present magneto-optical disc 5, the user zone is divided into16 zones, as described above, and the number of data bytes per segment(bytes/seg) and the number of segments per sector (seg/sector) aredetermined by the servo clock sclk times M/N (where M and N areintegers). That is, if the number of servo clocks sclk in the servo areaARs is N, the integer variable M which is specified zone by zone isdefined as DCLK shown in FIG. 10, and the number of data clocks equalsthe number of servo clocks times M/N, the number of servo clocks persegment (SCLKseg) and the number of data clocks per segment (DCLKseg)are given by:

    SCLKseg=9N

    DCLKseg=(SCLKseg)×(M/N).

Each track is divided into 1400 segments, as described above. Of these,1300 are data segments DSEG. Since no data are recorded in the GCP band,100 of the 1300 data segments are used as GCP segments GCPseg forrecording the GCP information, such as media information. The GCPsegment GCPseg is allocated to a data segment at a mid position of eachaddress segment ASEG, as shown in FIG. 13. The GCP segment GCPseg ismade up of the servo area ARs and the GCP area ARgcp located between twoblank areas ARblk, as shown in FIG. 14. As in the address segment ASEG,there are recorded in the GCP area ARgcp seven 4-bit data, that is[GCPH], [GCP2], [GCP3], [GCPL] and parity [P] and page numbers [PNH] and[PNL], as pits in the Gray code in the same way as the access code. TheGCP codes permit error detection due to appendage of parity [P]. Inaddition, plural sorts of media information may be afforded as the GCPinformation by the addition of the page number [PNH] and [PNL]. If thevolume is less than 16 pages, the same information may be recorded in[PNH] and [PNL] for invulnerability to errors.

By arranging each GCP segment GCPseg in a state of coincidence with thepage number of the GCP segment GCPseg of the lowermost bit of theaddress recorded in the address segment ASEG (frame number), misreadingof the frame number of the address segment ASEG or the page number ofthe GCP segment GCPseg may be eliminated. Also, by iteratively recordingten sorts of the GCP information ten times, misreading of the ten sortsof the GCP information may be diminished.

Referring again to FIG. 1, the operation of the recording/reproducingapparatus for the magneto-optical disc of the illustrated embodimentwill now be explained. An exchange of commands and data is executed withthe host computer 3 connected via the SCSI bus 4. Processing for theexchange of commands and data is controlled by a controller 21 of thecontrol circuit block 2. The controller 21 appends CRC and errorcorrection codes to data from the host computer 3 during recording andtransfers the data to the disc drive 1. For reproduction, data from thedisc drive 1 is corrected for errors and only user data portions aretransferred to the host computer 3. Commands to the servo system andrespective blocks of the disc drive 1 are given by the DSP 47 whichperforms necessary processing responsive to commands from the controller21.

Specifically, the DSP 47 is responsive to a request from the hostcomputer 3 in the state in which the magneto-optical disc 5 is loaded bythe loading unit 8 on a turntable to command the spindle driver 7 viaI/O block 46 to rotate the spindle motor 6. The DSP 47 may also issue asimilar command when the magneto-optical disc 5 is loaded when theautomatic spin-up mode is set. When the spindle motor 6 reaches apre-set rpm, the spindle driver 7 issues a lock signal to apprise theDSP 47 that the rotation has been stabilized. During this time, the DSP47 shifts an optical pickup 9 by the pickup driver 44 towards the outerrim and the inner rim so that the pickup is positioned outside the userarea. If the focusing is captured in the user area, there is a risk ofinadvertent data erasure if the disc is a highly sensitive disc. Bycapturing focusing in an area outside the user area, such as the GCParea outside the recording area, inadvertent data erasure may beprohibited from occurring.

When the spindle motor 6 reaches a constant rpm and the pickup 9 ismoved to the vicinity of the outer rim, for example, the DSP 47 sets thebias current LDbias to the laser driver 11 via the D/A converter 24 fromthe I/O block 23 and issues a command to the servo timing generator(STG) 32 to emit the laser light. The STG 32 controls the on/off of thelaser diode 10. This causes the laser beam to be emitted from the laserdiode 10 and radiated on the magneto-optical disc 5.

The reflected laser beam is received by the photodetector 12 provided inthe pickup 9. The photodetector 12 forms MO and RF signals, based uponthe level of the received reflected light, and forms tracking errorsignals, focusing error signals and FAPC signals for controlling thelaser level of the laser beam. These signals are sent to the IVconversion block 13. The IV conversion block 13 converts the abovesignals, supplied as electrical current, into an electrical voltage. TheIV conversion block 13 sends the MO and RF signals to the selector andclamp circuit 27, while sending the tracking error signals, focusingerror signals and FAPC signals to the multiplexor 41, which thentime-divisionally selects the tracking error signals, focusing errorsignals and FAPC signals and sends the selected signals to the A/Dconverter 42. The A/D converter 42 digitizes the time-divisionallysupplied tracking error signals, focusing error signals and FAPC signalsbased upon servo clocks sclk from the SPLL circuit 29 as laterexplained, and sends the digitized signals to the DSP 47 via the I/Oblock 43 and the DSP bus 22.

The DSP 47 detects the laser level based upon the FAPC data and feedsback the light intensity control data calculated by a digital filter tothe laser driver 11 via the I/O block 23 and the D/A converter 24 forcontrolling the laser power of the laser diode to be constant. The DSP47 also causes the electrical current to flow from the PWM circuit 45through a focusing driver of the pickup driver 44 for verticallyactuating the focusing actuator of the pickup 9 for establishing thefocusing search state as later explained. The reflected light from themagneto-optical disc 5 is detected by the photodetector 12. A detectionoutput of the photodetector 12 is converted by the IV conversion block13 into an electrical voltage which is supplied as a focusing errorsignal to the multiplexor 41 via a matrix amplifier.

Similarly to the FAPC signal, the focusing error signal is digitized bythe A/D converter 42 as a signal time-divisionally selected by themultiplexor 41 so as to be supplied via the I/O block 43 to the DSP 47.The DSP 47 feeds back the focusing control data obtained on digitallyfiltering the digitized focusing error signals from the PWM circuit 45to the focusing driver of the pickup driver 44 for establishing thefocusing servo loop. On stabilization of the focusing servo, the RFsignals produced by the IV conversion block 13 from the detection outputof the photodetector 12 are rendered constant in amplitude to someextent and clamped to a suitable potential by the selector and clampcircuit 27 before being converted into digital signals by the A/Dconverter 28. The sampling clocks sclk are of a frequency in thefree-running state of the servo clock generating (SPLL) circuit 29. Forthe timing pulse during clamping, a signal obtained onfrequency-dividing the free-running frequency by a pre-set value by theSTG 32 is employed.

The SPLL circuit 29 checks the difference in amplitude of RF signalsdigitized by the A/D converter 28 to check a pit patterning in order tosearch for the same pattern as that of a pre-set pit row in the servoarea. If this pattern is found, the SPLL circuit 29 controls the clockselector 31 so that the window is opened at a timing of possibleappearance of the next pattern, and checks for possible patterncoincidence. If this operation is confirmed a pre-set number of times,the SPLL circuit 29 is deemed to be in a locked state with respect tothe disc. The phase information is obtained by adding the amplitudedifference at both edges of the wobble pit Pb or Pc in the servo area.The phase information data obtained from each of the two wobble pits Pbor Pc are summed for absorbing gain fluctuations caused by amplitudechanges ascribable to the tracking position.

When the SPLL 29 is locked, the segment-based position becomes definitesuch that the segment mark position also becomes recognizable. The clockselector 31 is controlled for opening windows at the four positions A,B, C and D shown in FIGS. 3A-3D and 4 for searching the maximumamplitude position among the RF signals sampled at the four positions A,B, C and D. If the result is A, it can be recognized that the bit is anaddress mark, with the segment being an address segment ASEG and beingat the leading end of the frame. Thus the frame counter may be clearedfor frame synchronization.

Since each frame is made up of 14 segments, the clock selector 31 iscontrolled so that a window is opened every 14 segments. If continuousrecognition as an address mark is possible, frame synchronization isjudged to be locked. Upon frame synchronization, the address recordingposition can be recognized, so that the track address and the frame codeare decoded by an address decoder (ADEC) 33. With the ADEC 33, thepattern of four bits coded in a Gray code is decoded by checking thecoincidence with the Gray code table shown in FIGS. 5C and 6. The ADEC33 samples the playback RF signals at positions a, b, c and d shown inFIG. 5C and finds the maximum amplitude position by a differentialdetection method. Similarly, the playback RF signals are sampled atrespective positions e, f, g and h shown in FIG. 5C and the maximumamplitude position is found in order to effect decoding by thecombination and the Gray code table. By the above process, the trackaddress [AM] to [AL], parity [P], and the frame address [FM] and [FL]are decoded and the decoded results are stored in a register (notshown).

When the data is established, the DSP 47 reads out the register fordetecting the current position of the pickup 9. Since it is not the fourbits but the entire pattern that is coded in Gray code, a comparisonwith an inverted table or a non-inverted table is executed dependingupon whether the LSB of the upper four bits is [1] or[0]. If, when theinitially decoded frame code is loaded in a frame counter, and a numberobtained on incrementing the frame counter on the frame basis iscompared to the actually reproduced frame code, continuous coincidenceis confirmed, rotation synchronization is deemed to be established. Byreturning the number obtained from the frame counter as the frame codeto the DSP 47, there is no risk of a mistaken recognition of the frameposition despite some defects.

The ADEC 33 decodes the GCP information in a similar manner to the trackaddress and the frame code. However, the contents of the register areread out based on the GCP segment GCPseg having the GCP informationrecorded thereon instead of on the address segment. Thus the contents ofthe GCP area ARgcp may be confirmed.

The DSP 47 calculates the speed of movement of the pickup 9 as it readsthe Gray coded track address for controlling the slide motor via a slidedriver of the pickup driver 44 from the PWM circuit 45 for shifting thepickup 9 to a target track. When the pickup 9 reaches the target track,the tracking operation is executed. The tracking error signal TE isobtained by taking a difference of the amplitudes of RF signalsreproduced from wobbling pits in the servo area. The DSP 47 feeds backthe tracking control data resulting from digitally filtering thedifference value to the galvano-driver and the slide driver of thepickup driver 44 via the PWM circuit 45 for constituting a trackingcontrolling servo loop. The slide motor is driven by the slide driverfor controlling variations in low-frequency components and thegalvano-motor of the pickup 9 is driven by the galvano-driver foreffecting tracking control so that the laser spot will be at the trackcenter.

The leading position of the target sector is detected under such atracking state. The sector mark is present in a segment at the leadingend of the sector and a segment directly preceding the segment. Therespective sector marks control the clock selector 31 for opening thewindows at the four positions A, B, C and D shown in FIG. 4. If themaximum amplitude position among the RF signals sampled at these fourpositions A to D is B, it specifies that the segment is the leading endsegment of the sector, whereas, if the maximum amplitude position is C,it specifies that the segment is the segment directly previous to thesector. Basically, the segment at the leading end of the sector isdetermined by converting the sector address given by the host computer 3into a physical sector and finding in which segment of which track thesector is located. The probability of the two kinds of sector marksbecoming simultaneously defective is empirically not higher than 10⁻¹⁰,such that the probability of occurrence of defective sectors isextremely small.

The data clock generating circuit (DPLL) 30 generates data clocks dclkobtained on multiplying the flame-synchronized servo clocks sclkobtained from the SPLL circuit 29 by M/N and sends the data clocks dclkto a timing generator (DTG) 35 and the recording/reproducing circuit 36.

The recording/reproducing circuit 36 is fed during the recording modewith recording data via the controller 21 from the host computer 3. Therecording/reproducing circuit 36 adds random numbers of e.g., 127periods to the recording data by exclusive OR (EXOR) for carrying outsector-based scrambling in accordance with Y=X⁷ +X+1 and modulates thescrambled recording data into NRZI data synchronized with the dataclocks dclk. The initial value is set to 1 for each segment. Themodulated signal wdat is fed via the magnetic head driver 15 to themagnetic head 14, which then generates a magnetic field corresponding tothe modulated signal wdat and applies the magnetic field to the dataarea ARd of the magneto-optical disc 5 superheated to the Curietemperature by the laser beam emitted by the laser diode 10 forrecording the NRZI data.

During playback mode operation, the playback MO signal obtained by theIV conversion block 13 from the detected output by the photodetector 12is converted into digital signals by the A/D converter 28, after beingclamped to a suitable potential by the selector and clamp circuit 27, soas to be supplied to the recording/reproducing circuit 36. Therecording/reproducing circuit 36 decodes the NRZI data by viterbidecoding after digitally filtering the playback MO signals digitized bythe A/D converter 28 in conformity to partial response (1,1). The NRZIdata is converted on the segment basis into the NRZ data which aredescrambled on the sector basis into playback data which is transmittedvia the controller 21 to the host computer 3.

By scrambling recording data in this manner, the data pattern israndomized so that the probability of the continuation of paths thatcannot be established at the time of viterbi decoding is small while thememory capacity may be reduced. In addition, since the bit array israndomized for the ROM disc, the bit presence/absence ratio on the discapproaches 50% thus facilitating disc molding.

When instructed by the host computer 3 via the controller 21 to startrecording or reproduction, the DSP 47 enters into a focus search mode ofeffecting tentative focusing on the disc surface prior to recording orreproduction. In the focus search mode, the DSP 47 operates inaccordance with the flow chart shown in FIG. 15. That is, the presentflow chart is programmed by software and starts when the main powersource of the recording/reproducing apparatus for the magneto-opticaldisc is turned on to proceed to step S1.

At step S1, the STG 32 discerns whether or not a start of recording orreproduction is specified by the host computer 3 by determining if adrive-on-command signal has been received. If the result is NO, the DSP47 repeats the step S1 and, if the result is YES, the DSP transfers tostep S2 where the DSP 47 controls the laser driver 11 to fire the laserdiode 10. The DSP then transfers to step S3.

At step S3, the DSP 47 actuates only the focusing servo. The fixedclocks, which are clocks for the fixed frequency during focus search,are selected and sent to the D/A converter 42 and to the counter 49.That is, with the recording/reproducing apparatus for themagneto-optical disc, the rpm of the magneto-optical disc 5 is 2400. Atthis time, the frequency of servo clocks formed by the SPLL circuit 29based on playback signals (RF signals) having a servo pattern shown inFIG. 3 is 12.096 MHz.

As shown in FIG. 16, the controller 21 is provided with a quartzoscillator 60 outputting clocks of 40 MHz and two 1/2 frequency dividers61, 62. The controller 21 divides the 40 MHz clocks from the quartzoscillator 60 by 1/2 by divider 61 and sends the resulting 20 MHz clocksvia output terminal 64 to a CPU, not shown. The controller also dividesthe 20 MHz clocks by 1/2 by divider 62 to form 10 MHz clocks which aresent via output terminal 63 to the DSP 47.

The DSP 47 has a 5-tuple circuit 66 and a 1/4 frequency divider 67 shownin FIG. 17. The DSP 47 converts the 10 MHz clocks supplied via inputterminal 65 from the controller 21 into 50 Mhz clocks by the 5-tuplecircuit 66 and frequency-divides the 50 MHz clocks by 1/4 by the 1/4frequency divider 67 to generate fixed 12.5 MHz clocks close to thefrequency (12.096 MHz) of the servo clocks. The 12.5 MHz clocks are sentvia output terminal 68 to the STG 32.

As shown in FIG. 18, the STG 32 has a changeover switch 71 and aflip-flop 73 for controlling the switching of the changeover switch 71.The changeover switch 71 has its input terminals 71a, 71b fed with servoclocks of 12.096 MHz from the SPLL circuit 29 via input terminal 69 andwith the 12.5 MHz fixed clocks from the DSP 47 via input terminal 70,respectively.

During the focus search, the DSP 47 writes switching data "1", which is1-bit switching data, in the flip-flop 73. If the focusing search is notperformed solely, the DSP 47 writes switching data "0" in the flip-flop73. The switching data written in the flip-flop 73 is supplied to thechangeover switch 71. The changeover switch 71, fed with the switchingdata "1" or "0", controls a movable contact 71c to select either theinput terminal 71b or the input terminal 71a, respectively. Thus, duringthe focusing search, the fixed clocks of 12.5 MHz are selected by thechangeover switch 71 so as to be supplied via output terminal 74 to theA/D converter 42 and to the counter 49. If the focusing servo is notperformed solely, the 12.096 MHz servo clocks are selected and suppliedvia output terminal 74 to the A/D converter 42, the DSP 47, and thecounter 49.

At step S3, the STG 32 switches in the manner described above, betweenfixed clocks and servo clocks during focus search before transferring tostep S4. At step S4, the focus search is carried out, before the DSPtransfers to step S5. That is, the A/D converter 42 is fed viamultiplexor 41 with focusing error signals formed for each servo area,as shown in FIG. 19a, and fixed clocks of 12.5 MHz. The A/D converter 42latches the focusing error signals, based on the fixed clocks, anddigitizes the latched signals to send the digitized data as the focusingerror signal via I/O block 43 to the DSP 47, as shown in FIG. 19b. TheDSP 47 is fed with interrupt pulses from STG 32, as shown in FIG. 19c.The DSP 47 captures focusing error data based upon the interrupt pulsesas shown in FIG. 19d and transmits the captured focusing error data tothe PWM circuit 45.

When both the focusing servo and the tracking servo are performed, theDSP 47 sets "A0h" (hexadecimal number for 160) in the focusing timingcontrol register 48, as will be explained subsequently. During focussearch or seek, the DSP sets "50h" (hexadecimal number for 80) in thefocusing timing control register 48. The counter 49 counts the number ofthe fixed clocks from the STG 32 during focus search.

The comparator 50 compares the setting value of the focusing timingcontrol register 48 to the count value of the counter 49 and sendsoutput timing control data shown in FIG. 19e to the PWM circuit 45 at atiming when the count value of the counter 49 is equal to a settingvalue "50h" of the focusing timing control register 48, that is at atiming when the count value becomes equal to 80. The PWM circuit 45outputs the focusing error data supplied from the DSP 47 each time theoutput timing data is supplied.

Consequently, during the focus search, focusing error data is outputtedfrom the PWM circuit 45 at a timing substantially equal to one-half thatwhen both focusing servo and tracking servo are performed. The focusingerror data are supplied via the pickup driver 44 to the pickup 9. Thepickup 9 vertically shifts an objective lens based upon focusing errordata for tentative focusing.

Referring again to FIG. 15, at the next step S5, the DSP 47 judges,based upon focusing error data produced during the focus search, whetheror not the laser beam is correctly focused on the surface of themagneto-optical disc 5. If the result is YES, the DSP transfers to stepS6 and if otherwise to step S9. At step S9, it is judged whether or nota pre-set time has elapsed since the start of focus search, that iswhether a time-out has occurred. If the result is NO, the DSP repeatsthe routine of steps S5 and S9. If the result is YES, the DSP deems thatsome trouble has occurred and directly terminates the routine shown inthe flow chart of FIG. 15.

At step S6, the DSP 47 judges whether or not the PLL circuit for thefocusing servo is locked. If the result is YES, the DSP transfers tostep S7 and if the result is NO, the DSP transfers to step S10. At stepS10, it is judged whether or not a pre-set time has elapsed since thestart of the focusing lock, that is whether a time-out has occurred. Ifthe result is NO, the DSP repeats the routine of steps S6 and S10. Ifthe result is YES, the DSP deems that some trouble has occurred anddirectly terminates the routine shown in the flow chart of FIG. 15.

At step S7, since the focusing servo capturing is terminated, the DSP 47changes the switching data of flip-flop 73 of FIG. 18 from "1" to "0"before transferring to step S8. Thus the input terminal 71a of thechangeover switch 71 is selected by its movable contact 71c and the12.096 MHz servo clocks from the SPLL 29 are fed via the changeoverswitch 71 to the A/D converter 42 and the counter 49. Focus servo isperformed based upon focusing error data formed by the A/D converter 42by the 12.096 MHz servo clocks.

The DSP 47 starts the tracking servo at step S8 to terminate the entireroutine of focus search shown in FIG. 15. When the focus servo comes toa close, the recording/reproducing apparatus for the magneto-opticaldisc enters into the normal servo control mode of simultaneouslyapplying, the focusing servo and tracking servo.

In the normal servo control state, the DSP 47 sets "A0h", a hexadecimalnumber for 160, in the focus timing controlling register 48, whilesetting "80h", a hexadecimal number for 128, in a tracking error timingcontrol register, not shown. A laser beam is radiated on themagneto-optical disc 5 for reproducing the servo area and the data areain this order as shown in FIG. 21a, so that the focusing error signaland the tracking error signals are sent via multiplexor 41 to the A/Dconverter 42, as described above. The A/D converter 42 digitizes thefocusing error signals and the tracking error signals, based upon servoclocks supplied from the SPLL circuit 29 (via the STG 32), in order tooutput the focusing error and tracking error.

For improving tracking servo response characteristics, the DSP 47captures tracking error data by tracking error latch pulses suppliedfrom STG 32 as shown in FIG. 21b, prior to engaging focusing error data,and then captures focusing error data by a focusing error latch pulseshown in FIG. 21c. The DSP 47 effects phase compensation on the focusingerror and tracking error signals in this order as shown in FIG. 21ebased upon sampling interrupt pulses supplied from STG 32 as shown inFIG. 21d and transmits the resulting data to the PWM circuit 45.

Specifically, the rpm of the disc in the recording/reproducing apparatusfor the magneto-optical disc is 2400, while the frequency of servoclocks from the SPLL circuit 29 is 12.096 MHz. On the other hand, eachsegment is of a period 216 times that of the servo clocks, so that thesampling clocks for the focus error signals employed in the A/Dconverter 42 are 56 kHz. The filter coefficients for phase compensationare pre-calculated based upon this frequency and stored in a memoryprovided in the DSP 47. Thus, when engaging the focusing error andtracking error signals, the DSP 47 phase-compensates the error databased upon the filter coefficients stored in the memory and transmitsthe resulting data to the PWM circuit 45.

The counter 49 is fed with servo clocks from the SPLL circuit 29 andsends the count value to the comparator 50. The value "80h", set in thetracking timing control register, is fed to the comparator 50. When thecount value from the counter 49 is "80h", that is when the count valueis 128, the comparator 50 sends output timing data to the PWM circuit45, which outputs the tracking error at a timing when the output timingdata is supplied thereto. This causes the PWM circuit 45 to outputtracking error as shown in FIG. 21f.

The value "80h", set in the tracking timing control register duringnormal servo control, is fed to the comparator 50. When the count valuefrom the counter 49 is "A0h", that is when the count value is 160, thecomparator 50 sends output timing data to the PWM circuit 45, whichoutputs the focusing error at a timing when the output timing data issupplied thereto. This causes the PWM circuit 45 to output the trackingerror data as shown in FIG. 21g.

The tracking error data and the focusing error data are fed to thepickup driver 44. The pickup driver 44 drives the pickup 9 in adirection of correcting the tracking error and the focusing errorspecified by the tracking error data and the focusing error data. Thisenables the recording data to be correctly reproduced at all times inthe just-tracking state and in the just-focusing state.

Referring to the flow chart of FIG. 20, the operation of therecording/reproducing apparatus for the magneto-optical disc during seekis explained. With the flow chart shown in FIG. 20, the program isstarted when the man power source of the recording/reproducing apparatusfor the magneto-optical disc is turned on, and transfers to step S11. Atstep S11, the controller 21 sends a seek command received from the hostcomputer 3 to the DSP 47. The DSP 47 then transfers to step S12.

Since the seek is specified at step S12, the DSP 47 sets the PLL circuitfor the tracking servo to the state of an open loop. Since the focusingservo is executed alone, the DSP 47 sets "50h" in the focusing timingcontrol register 48 at step S14. At step S14, the DSP 47 causes thepickup 9 to be moved via the PWM circuit 45 and the pickup driver 44 andexecutes seek as far as the target track. The DSP then transfers to stepS15.

At step S15, the DSP executes the focusing servo capture at a timing of"50h" as set in the focusing timing control register 48, as explainedabove. When seek is completed, the DSP 47 then transfers to step S16. Atstep S16, the DSP 47 sets the PLL circuit for tracking servo to thestate of Closed loop to terminate the entire routine of the flow chartshown in FIG. 20. On termination of the seek operation, the DSP 47 sets"80h" and "A0h" in the tracking timing control register and in thefocusing timing control register 48 as explained above for executingservo control comprised of both the tracking servo and the focusingservo.

It will be clear from the foregoing that, with the MOrecording/reproducing apparatus of the present invention, since therespective parts are controlled so that the focusing error data will beoutputted earlier than with the usual timing when the focusing servo iscarried out by itself, such as during focus search or seek, it becomespossible to reduce the time duration between latching of the focusingerror signals from the A/D converter 42 until outputting thereof via thePWM circuit 45. The result is that phase allowance of the servo loop maybe improved and the focusing servo which tends to become unstable duringfocusing search and seek may be stabilized for shortening the timerequired until the start of recording or reproduction. When the focussearching alone is carried out, the focusing servo is captured usingfixed clocks of a fixed frequency and, once the focusing servo iscaptured, the fixed clocks are switched to usual servo clocks forcarrying out the focusing servo.

Ordinarily, if the PLL for carrying out focusing servo is in thefree-running state, the frequency of the servo clocks outputted by avoltage controlled oscillator (VCO) is fluctuated by approximately "30%.It takes a time of from several msec to tens of msec after focusingservo is captured until the PLL is locked. If a closed loop isestablished by capturing the focusing servo in this state and thefocusing servo is performed for phase compensation, there is produced anerror in the sampling frequency due to clock variation, such that filtercoefficients for phase compensation become incorrect and phasecompensation becomes unfeasible. In particular, the focusing actuatoramplitude is large directly after capture of the focusing servo so thatthe servo state tends to become unstable and hence the focusing servo,once captured, becomes disengaged due to troubles in phase compensation.However, by employing fixed clocks of the fixed frequency during thefocusing servo engagement as in the present recording/reproducingapparatus for the magneto-optical disc, phase compensation directlyfollowing the focusing servo engagement may be rendered more correct forrealizing stable the focusing servo.

The fixed clocks are of 12.5 MHz which is slightly different from 12.096MHz of commonly employed servo clock frequency. However, the fixedclocks are free from frequency variations as compared to thefree-running servo clocks so that the above effect may be achievedwithout any inconveniences. The fixed clocks of 12.5 MHz are formed byfrequency division of 40 MHz from the quartz oscillator 60 shown in FIG.16. Although plural oscillators provided within the equipment might leadto increased cost and the problem of noise, these problems are avoidedin the recording/reproducing apparatus of the present invention byemploying only one quartz oscillator 60. Thus, noise is prohibited frombeing produced while the cost is lowered.

The phase compensation is executed when tracking servo and focusingservo are used in combination. However, it is also possible to providefilter coefficients for fixed clocks in advance and store them in thememory of the DSP 47 and to effect phase compensation of the fixedclocks using the filter coefficients of the fixed clocks during focusingsearch or seek (the filter coefficients are switched between combineduse of tracking servo and focusing servo and use only of focusingservo). This renders phase compensation of fixed clocks or servo clocksperformed by the DSP 47 more correct and allows achievement of a morestabilized focusing servo.

In the above description of the preferred embodiment, the output timingsof the tracking error data and the focusing error data during normalrecording and reproduction are set to 80h and A0h, respectively, whilethe output timing of the focusing error data in case the focusing servois effected by itself is 50h. However, any optional output timing may beused depending on design conditions. However, the output timing of thefocusing error data in the case that the focusing servo is executed byitself should be set in consideration that, if the focusing error dataare outputted too promptly, there is insufficient allowance in dataprocessing in the DSP 47.

Although the focusing servo signal control apparatus according to thepresent invention is applied in the above description to arecording/reproducing apparatus for a magneto-optical disc, it may beapplied to any other reproducing apparatus, recording apparatus orrecording/reproducing apparatus for an optical disc, such as a compactdisc or a write-once disc, provided that the apparatus operates on thesample servo system.

It is possible with the servo signal processing apparatus of the presentinvention to improve phase allowance of the focusing servo loop when thefocusing servo loop is in operation by itself during engagement offocusing or seek. The result is that stable focusing servo may berealized while the time required for engagement of the focusing servomay be shortened. Thus the time required for recording or reproductionmay be shortened if the servo signal processing apparatus is provided inthe reproducing apparatus, recording apparatus or recording/reproducingapparatus for an optical disc.

It is further possible with the focusing servo control device of thepresent invention to assure correct phase compensation directly afterthe focusing servo engagement to realize stable focusing servo. On theother hand, since fixed clocks are formed by e.g., frequency division ofsystem clocks of the pre-set frequency from the oscillating means forsystem clocks, the oscillating means for system clocks may besimultaneously used as oscillating means for fixed clocks. The result isthat it becomes possible to prohibit errors caused by provision ofplural high-frequency oscillating means, while the focusing servocontrol device may be lowered in cost through reducing the number ofcomponents and simplifying the construction.

Although the present invention has been shown and described with respectto the preferred embodiments, various changes and modifications may bemade within the technical concept of the present invention and aredeemed to lie within the spirit and scope of the invention as claimed.

What is claimed is:
 1. An apparatus for reproducing signals recorded onan optical disc having a plurality of equal angular servo areas eachhaving servo patterns for providing control of tracking, focusing andclocks, the apparatus comprising:drive means for rotating the opticaldisc and including an optical pickup for tracing a track formed on theoptical disc, the drive means reproducing signals from the rotatingoptical disc; servo clock generating means for deriving servo clocksfrom the reproduced signals output by the drive means; servo controlmeans for controlling the focusing and tracking of the optical pickupbased upon the servo clocks supplied therewith so as to establish afocusing servo and a tracking servo; fixed clock generating means forproducing fixed clocks so that the servo control means performs afocusing search based upon the fixed clocks; switching means forselectively supplying either the servo clocks or the fixed clocks to theservo control means; and switching control means for controlling theswitching means so that the fixed clocks, as a substitute for the servoclocks, are supplied to the servo control means in the focusing search,and for controlling the switching means so that the servo clocks aresupplied to the servo control means after engagement of the focusingservo.
 2. The apparatus of claim 1, further comprising analog to digitalconversion means for converting analog reproduced signals correspondingto the servo pattern into digital signals; andwherein the servo clockgenerating means compares the digital signals to a pre-stored servopattern and predicts a next timing of coincidence between the digitalsignals and the pre-stored servo pattern based on a timing when thedigital signals and the pre-stored servo pattern have been coincident.3. The apparatus of claim 2, wherein the servo clock generating meanscaptures the digital signals and again compares newly captured digitalsignals of the servo pattern to the pre-stored servo pattern.
 4. Theapparatus for claim 3, wherein the servo clock generating means regardsa synchronization of the reproduced signals from the optical disc is ina locked state when a coincidence between the digital signals and thepre-stored servo pattern has been continuously detected a predeterminednumber of times.
 5. The apparatus of claim 1, wherein the fixed clockgenerating means includes oscillating means for producing system clockshaving a predetermined frequency and frequency dividing means fordividing the system clocks so as to form the fixed clocks.
 6. Theapparatus of claim 1, wherein the servo pattern in each servo areaincludes a tracking portion used for the tracking control and a focusingportion used for the focusing control, the focusing portion followingthe tracking portion in the servo area.
 7. The apparatus of claim 1,wherein the servo control means includes means for digitizing a focusingerror signal and a tracking error signal supplied therewith from thedrive means based on the servo clocks, and means for effecting phasecompensation on the focusing error signals and the tracking errorsignals so as to output focusing error data and tracking error data inthe order in which they are supplied, and pickup driving means fordriving the optical pickup in a direction of correcting tracking errorand focusing error specified respectively by the tracking error data andfocusing error data at a timing corresponding to the tracking controland the focusing control.
 8. The apparatus of claim 7, furthercomprising timing control means for controlling the output timing of thefocusing error data so that, when the focusing control is effected byitself, the output timing is prompted at a predetermined time in orderto supply only the focusing error data to the pickup driving means. 9.The apparatus of claim 8, wherein the timing control means controls theoutput timing of the focusing error data during the focusing search forfocusing on the surface of the optical disc in the case of effecting thefocusing control by itself.
 10. The apparatus of claim 9, wherein, whenthe focusing search comes to a close, the timing control means controlsthe output timing of the focusing error data so as to succeed the outputtiming of the tracking error data.
 11. The apparatus of claim 8, whereinthe timing control means controls the output timing of the focusingerror data during a seek operation in which the optical pickup is movedto a desired track radially in relation to the optical disc by thepickup driving means.
 12. The apparatus of claim 11, wherein, when theseek comes to a close, the timing control means controls the outputtiming of the focusing error data so as to succeed the output timing ofthe tracking error data.
 13. In a recording/reproducing apparatus forrecording/reproducing an optical disc, a focusing servo operationcontrolling device of a phase-locked loop configuration in which afocusing servo operation is effected based upon focusing servo clocksformed by a playback output of pre-set servo patterns from the opticaldisc on which the servo patterns are arrayed at pre-set intervals,comprising:fixed clock outputting means for outputting fixed clocks fora focusing servo operation of a fixed frequency; switching means forswitching between outputting the focusing servo clocks or the fixedclocks; and control means for effecting switching control of theswitching means for selecting and outputting the fixed clocks wheneffecting the focusing servo operation alone and selecting andoutputting the focusing servo clocks directly after engagement of thefocusing servo operation.
 14. The apparatus as claimed in claim 13further comprising:analog to digital conversion means for analog todigital converting the playback output of the servo pattern; andcomparator means for comparing playback data of the servo patterns fromthe analog to digital converting means to a pre-stored servo pattern,predicting a next timing of coincidence between the playback data of theservo pattern and the pre-stored servo pattern based upon a timing whenthe playback data and the pre-stored servo pattern has been coincident,capturing the playback data of the servo pattern and again comparingnewly captured playback data of the servo pattern to the pre-storedservo pattern, the above sequence being repeated a pre-set number oftimes; the phase locked loop configuration being such that a phase lockwith respect to the surface of the optical disc is deemed to be achievedwhen the coincidence has been continuously detected a pre-set number oftimes by the comparator means.
 15. The apparatus as claimed in claim 13wherein the fixed clock outputting means includes oscillating means foroutputting system clocks of a pre-set frequency and frequency dividingmeans for frequency dividing the system clocks from the oscillatingmeans for forming the fixed clocks.
 16. The apparatus as claimed inclaim 15 further comprising phase compensation means having a firstfilter coefficient for fixed clocks and a second filter coefficient forfocusing servo clocks, the phase compensation means outputting the fixedclocks after phase compensation thereof with the first filtercoefficient, the phase compensation means outputting the focusing servoclocks after phase compensation thereof with the second filtercoefficient; andthe control means effecting switching control of theswitching means for selecting and outputting the fixed clocks, thecontrol means controlling the phase compensation means for effectingphase compensation with the first filter coefficient, when effecting thefocusing servo operation alone, the control means effecting switchingcontrol of the switching means for selecting and outputting the focusingservo clocks, the control means controlling the phase compensation meansfor effecting phase compensation with the second filter coefficient,after engagement of the focusing servo operation.